The present invention relates to an antenna device and a shield cover. More particularly, the present invention relates to a structure of a shield cover of a personal type small antenna device used for a digital radio receiving set capable of listening to digital radiobroadcasting when electric waves sent from an artificial satellite, which will be referred to as satellite electric waves hereinafter, or electric waves on the ground, which will be referred to as ground electric waves hereinafter, are received.
Recently, digital radio receivers operable to receive the satellite electric waves or the ground electric waves and capable of listening to the digital radiobroadcasting. These digital radio receivers have already been actually put into practical use in the United States. In general, these digital radio receivers are mounted on mobile bodies such as automobiles and capable of receiving electric waves, the frequency of which is in the frequency band of 2.3 GHz, so as to listen to the radiobroadcasting. That is, the digital radio receivers are radio receivers capable of listening to mobile radiobroadcasting. Since the frequency of the electric waves to be received is in the frequency band of about 2.3 GHz, the wave-length (the resonance wave-length) λ of the electric waves to be received is about 128.3 mm. In a case where these digital radio receivers receive the ground waves, the ground waves are transmitted in such a manner that satellite waves are once received by a global station and then transmitted again by linear polarization after the frequency has been a little shifted. The satellite electric waves are of the circular polarization. On the other hand, the ground electric waves are of the linear polarization.
In this digital radiobroadcasting, electric waves are used, the frequency of which is in the frequency band of about 2.3 GHz. Therefore, in order to receive the electric waves, it is necessary to install an antenna device outdoors.
These digital radio receivers includes a receiver mounted on automobiles, a receiver installed in houses, and a portable receiver, the electric power sources of which are batteries. A specific example of the portable digital radio receiver is a portable electronic device such as a portable acoustic device. This portable electronic device includes not only a digital tuner used for listening to digital radiobroadcasting but also an optical disk drive for regenerating an optical disk such as a compact disk (CD), an amplifier and a speaker. These components are integrally built in a housing.
On the other hand, various structures of antennas are provided as the antennas capable of receiving electric waves, the frequency band of which is about 2.3 GHz. The antenna can be divided into two main types by its shape. One is a plane type (a flat plate type) antenna which is a patch antenna and the other is a cylindrical antenna such as a loop antenna or a helical antenna. The plane type antenna and the cylindrical type antenna described above are prepared separately from the housing of the portable electronic device and connected to the digital radio tuner, which is built in the housing through a cable and a connector, and used.
The antenna device used for satellite radiobroadcasting includes: an antenna case; and an antenna module accommodated in the antenna case. The antenna module includes: a circuit board having a primary side and a reverse side; a plane antenna element such as a patch antenna mounted in the primary side of the circuit board; an LNA (low noise amplification) circuit mounted on the reverse side of the circuit board; and a shield cover for shielding the LNA circuit.
In a plan view, the shield cover is formed into a substantial rectangle, the four corners of which are chamfered. That is, the shield cover has corner portions at the four corners. The shield cover is attached to a reverse side of the circuit board when the four corners (the corner portions) are soldered.
A related-art shield cover 21 will be explained in detail with reference to the FIGS. 1(A) to 1(F) and FIGS. 2(A) to (C). As shown in FIG. 1(A), the shield cover 21 is composed symmetrically with respect to center line A-A of FIG. 1(B). The shield cover 21 includes: a substantially rectangular ceiling portion 22, the four corners of which are chamfered; and four side wall portions 23, 24, 25, 26 which are formed on four sides of outer periphery of the ceiling portion 22. The four side wall portions 23, 24, 25, 26 are respectively referred to as a front side wall portion, a rear side wall portion, a right side wall portion and a left side wall portion. The front side wall portion 23 is composed of a pair of front side pieces which are separate from each other with respect to center line A-A. An opening 27, into which a coaxial cable is inserted, is provided between both the front wall pieces. As shown in FIGS. 1(B), 1(D), and 1(F), a soldering portion 28, to which an earth line of the coaxial cable is soldered, is formed at a position opposed to the opening 27 in the ceiling portion 22. Further, the shield cover 21 includes corner portions 29 which are formed at four corners. In both end portions 25, 26, protruding portions 30, 31, used for fixing the shield cover 21 to a base which is referred to as “a bottom cover”, are formed.
When the wall portions 23, 24, 25, 26 and the corner portions 29 are bent by a right angle with respect to the ceiling portion 22, the shield cover 21 shown in FIG. 1(A) is composed.
This shield cover 21 is soldered onto a reverse side of the circuit board at the corner portions 29. At this time, as shown in FIGS. 1(C), 1(E), 2(B) and 2(C), the protruding portions 32, 33, which are formed in both side wall portions 25, 26, are respectively inserted into the holes formed on the circuit board.
In this connection, a shielding case for shielding the board arranged inside is described in Japanese Patent Publication No. 08-293688A, for example.
However, in the case of the related-art shield cover 21, the following problems may be caused. As shown in FIGS. 1(A) and 2(A), at the bent portions (the bent portions between four side wall portions 23 to 26 and the corner portions 29) of the shield cover 21, gaps G are generated. As a result, the ground property of this shield cover 21 is low.
In this connection, the shielding case described in Japanese Patent Publication No. 08-293688A is used for shielding the board arranged inside. Therefore, the structure of the shielding case described in Japanese Patent Publication No. 08-293688A is different from that of the shield cover attached onto the reverse side of the circuit board for shielding a circuit such as an LNA (low noise amplifying) circuit mounted on the reverse side of the circuit board.